The present invention relates to a spatial light modulator applicable to, for example, an image pickup device and an optical projector, or employed on, for example, an optical computer and a video projector.
For receiving an optical image and emitting the optical image for an optical projector and for optical parallel signal processing in an optical computer and image-recording, a spatial light modulator element, such as a liquid crystal-type optical modulator, a photoconductive Pockels-effect device and a microchannel spatial light modulator or a device made of a photochromic material has been developed.
One known spatial light modulator is shown in FIG. 1. The spatial light modulator is composed of glass plates 10 and 12, transparent electrodes 14 and 16, a photoconductive layer 18, a light blocking layer 20, a dielectric mirror 22, and a photo-modulation layer 24 composed of twisted-nematic liquid crystals in which the intensity of the light incident thereto varies accordingly with the field distribution applied thereto. The photo-modulation layer 24 is formed by filling the twisted-nematic liquid crystals into a cell formed by spacers 30a and 30b and sandwiched between aligning layers 26 and 28.
In FIG. 1, when a light WL carrying an optical image is incident to the spatial light modulator through the glass plate 10 under the state that an a.c. voltage is applied across the transparent electrodes 14 and 16 by an a.c. power supply 32 and reaches the photoconductive layer 18, the electric resistance thereof varies accordingly with the optical image carried by the light WL.
A charge image corresponding to the optical image is thus generated in the vicinity of the border of the photoconductive layer 18 and the light blocking layer 20. At this time, a field distribution corresponding to the charge image is applied to the photo-modulation layer 24.
Then, when a linearly polarized reading light RL is incident to the spatial light modulator through the glass plate 12, the light RL propagates through the transparent electrode 16, the aligning layer 28, the photo-modulation layer 24, the aligning layer 26, the dielectric mirror 22 and the light blocking layer 20 in succession.
Most of the light RL returns to the glass plate 12 as reflected light from the dielectric mirror 22. In this case, the optical axes of molecules of the liquid crystals in the photo-modulation layer 24 become not parallel to the transparent electrodes 14 and 16. A condition of double refraction of the light RL varies accordingly with the field intensity applied to the photo-modulation layer 24 due to the electro-optic effect of the liquid crystals. A reflected light is thus generated, whose plane of polarization varies according to the generated charge image. As a result, an optical image information corresponding to the optical image carried by the light WL is generated at the glass plate 12.
The light component of the light RL which is not reflected at the dielectric mirror 22 is blocked by the light blocking layer 20 from reaching the photoconductive layer 18. The electric resistance thereof thus does not vary, so that the charge image generated in the vicinity of the border of the photoconductive layer 18 and the light blocking layer 20 also does not vary.
Therefore, a circularly polarized light emitted from the glass plate 12 is passed through an analyzer (not shown) to have a spatial intensity distribution corresponding to the optical image carried by the light WL.
However, there are problems in this conventional image convertor. Namely, it requires a complex process to from the photo-modulation layer 24, that is, the twisted-nematic liquid crystal must be filled into the cell composed by the spacers 30a and 30b.
Furthermore, it is difficult to compose a photo-modulation layer 24 by means of twisted-nematic liquid crystal with uniform thickness in order to produce large spatial light modulators.
It is even more difficult to read and write an image with high resolution by this conventional spatial light modulator.
Next, a photo-modulation element (spatial light modulator) is capable of incoherent to coherent optical conversion or vice versa. Thus, the element is applicable to, for example, data parallel processing and image real-time processing and also to a display system of a video projector by way of optical intensity amplification.
Japanese Patent Laid-Open Application No. 58(1982)-215626 and Appl. Phys. Lett., Vol. 22, No. 3, Feb. 1, 1973 disclose such an element as shown in FIG. 2.
The element shown in FIG. 2 is composed of glass plates 10 and 12, transparent electrodes 14 and 16, a photo-modulation layer 24 in which liquid crystals are used as the modulation material, a photoconductive layer 18, a dielectric mirror 22 and a nonconductive light blocking membrane 20a.
When a light WL carrying information is incident to the element through the glass plate 10, the light WL reaches the photoconductive layer 18. Pairs of electrons and holes are generated in the photoconductive layer 18 correspondingly with the intensity of the light WL when an a.c. voltage is applied across the transparent electrodes 14 and 16. The pairs are separated from each other to generate a charge image corresponding to the intensity distribution of the light WL.
Next, a light RL for reading the information is incident to the element through the glass plate 12. When the light RL reaches the photo-modulation layer 24, the light RL is photo-modulated therein correspondingly with the intensity of the light WL, due to the electric field of the charge image generated in the photoconductive layer 18. The light RL thus modulated is reflected at the dielectric mirror 22 and emitted out from the glass plate 12.
One of the glass plates 10 and 12 or both of them may be eliminated if crystallized liquid crystal or liquid crystal incorporated in a film is employed as the photo-modulation layer 24. The light blocking layer 20a prevents a part of the reading light passed through the dielectric mirror 22 from reaching the photoconductive layer 18, which would disturb the charge image generated therein and degrade the contrast ratio of the image to be read out.
As is described, a reflecting mirror for reflecting a light RL cannot be eliminated in the photo-modulation element to read an image by way of reflection. A conductive material such as a metal layer cannot be employed as the reflection mirror in principle, so that the dielectric mirror 22 is employed.
FIG. 3 shows the dielectric mirror 22 which is composed by alternately laminating layers with low refractive index and layers with high refractive index, each having a thickness of .lambda./4 with respect to the wavelength .lambda. of the light RL. In FIG. 3, layers 22a made of SiO.sub.2 with low refractive index and layers 22b made of TiO.sub.2 are alternately laminated to each other, with one layer 22a with thickness of .lambda./2 at an outermost portion of the dielectric mirror 22.
The dielectric mirror 22 thus formed improves its reflectivity or degrades its transmittance of the light RL as the number of laminated layers increases. However, the reflectivity is degraded with an increase of the transmittance if the wavelength in the spectral characteristics of the dielectric mirror 22 falls out of the desired range.
Therefore, the light blocking layer 20a must be laminated to the dielectric mirror 22 shown in FIG. 3 if the light RL is not a monochromatic light and even if the light RL is low in intensity. Improvement in the reflectivity with increase in number of the layers to be laminated results in the thickness of the dielectric mirror 22 bring several microns or more and causes low productivity. A light blocking layer with a thickness of 1 to 10 microns must be employed on, for example, a video projector to emit an intense light RL.
As is described, a conventional photo-modulation element, particularly for a video projector, requires a thick dielectric mirror 22 with a thick light blocking layer 20a. This results in considerable loss of the electric field applied to the photo-modulation layer 24 from the charge image generated in the photoconductive layer 18, causing a lower contrast ratio of the image to be read out. Furthermore, the electric field leaking out in a transverse direction lowers the resolution of the image to be read out. Therefore, an upgraded power supply 32 must be employed.